This invention relates to a method and apparatus for guiding pliable sheet material so as to allow a forward portion of the sheet material to be continuously fed forward while a rear portion of the sheet material is stationary.
Apparatus such as this is of particular application in reprographic machines where the forward edge of a continuous strip of photo-sensitive copying paper is aligned with the forward edge of a master sheet before feeding the two forward edges into exposure and developing stations of the reprographic machine. As the reprographic processes normally involve continuous movement of the master sheet and registering copying paper through the machine, cutting the paper at a rear edge which will coincide with the rear edge of the master sheet must be effected while the forward edges of the master sheet and the copying paper are still moving.
In practice, rear edge cutting of the copying paper may be effected by forming the copying paper into a loop between the cutting means and the exposure and developing stations of the machine. The rear portion of the copying paper may then be held stationary and the rear edge cut while the forward edges of the copying paper and the master sheet move forward together and the loop is dissipated.
In one form of loop-forming feed apparatus suitable for use in reprographic machines, a master sheet and a continuous strip of copying paper are fed through converging paper guides into a common paper guide in which the two layers can be fed into the exposure and developing stations of a reprographic machine. A first sensing device disposed along the path of the master sheet detects the passage of the forward edge of the master sheet and is operable to initiate withdrawal of the copying paper from a rotatably mounted continuous roll and to feed the copying paper between upstream and downstream pairs of co-operating transport rollers so that the forward edge of the copying paper is moved into alignment with the forward edge of the master sheet at the junction of the converging paper guides and the two forward edges are fed together along the common paper guide.
A second sensing device disposed along the path of the master sheet, upstream of the first sensing device, then detects the passage of the rear edge of the master sheet and is operable to cause acceleration of the upstream pair of transport rollers engaging the copying sheet. Then, for a predetermined period, the downstream pair of transport rollers engaging the copying paper continue to rotate at a constant speed, consistent with the uniform motion of the forward edges of the master sheet and the copying paper, while the upstream pair of rollers rotate at a higher speed to form a loop of copying paper between the two pairs of transport rollers. The upstream pair of transport rollers is then stopped and a stationary rear portion of the copying paper held by the upstream pair of transport rollers is cut upstream of the two pairs of transport rollers while the downstream pair of transport rollers continue their rotation at constant speed so as to feed copying paper in the loop forward. The predetermined loop-forming period, in which the upstream and downstream pairs of transport rollers rotate at different speeds, is so controlled that the rear edge of the copying paper cut from the stationary rear portion of the sheet material will coincide with the rear edge of the master sheet.
This apparatus has the disadvantage that it involves the use of sophisticated and expensive means for controlling the rotation of the upstream pair of transport rollers so as to ensure that the rear edge of the piece of copying paper severed from the continuous strip of copying paper coincides precisely with the rear edge of the master sheet when these rear edges are brought together.
In an alternative type of loop-forming feed apparatus devised in order to avoid this disadvantage, the copying paper withdrawn from a rotatably mounted continuous roll is fed between upstream and downstream pairs of co-operating transport rollers. As before, the master sheet and the copying paper are fed along converging paper guides to a common paper guide extending from the junction of the converging paper guides, and the feed of the copying papers is initiated in response to the operation of a first sensing device which detects the passage of the leading edge of the master sheet. However, before the forward edge of the copying paper reaches the junction of the converging paper guides, the downstream pair of transport rollers is stopped so that a loop of copying paper is formed between the upstream and downstream pairs of transport rollers as a result of the continued rotation of the upstream pair of transport rollers. The downstream pair of transport rollers is then restarted in response to the passage of the forward edge of the master sheet past a second sensing device disposed along the path of the master sheet, downstream of the first sensing device, so that the forward edge of the copying paper is moved into alignment with the forward edge of the master sheet when these two forward edges reach the junction of the converging paper guides.
The copying paper is cut upstream of the two pairs of transport rollers when the upstream pair of transport rollers has been stopped in response to the passage of the rear edge of the master sheet past the first sensing device. The rear portion of the copying paper held stationary between the upstream pair of transport rollers is cut at any time up to and including the moment when the copying paper extending between the two pairs of transport rollers is drawn taut as a result of the continuous rotation of the downstream pair of transport rollers, but is preferably effected before the loop of this copying paper has been dissipated.
Thus, although this form of feeding apparatus avoids the use of complex timing and accelerating means, it still requires the use of relatively complicated and powerful means for stopping and starting the downstream pair of transport rollers.